Which color do Gram-negative bacteria stain after Gram staining?

Which color do Gram-negative bacteria stain after Gram staining?

• A. Purple
• B. Brown
• C. Pink
• D. Blue

Answer: (C)

Gram staining differentiates bacteria by cell wall structure. After the decolorization step, Gram-negative organisms lose the crystal violet–iodine complex because their thin peptidoglycan layer and outer membrane don’t retain it, so they take up the counterstain. The counterstain is typically safranin, which colors them pink. In contrast, Gram-positive bacteria have a thick peptidoglycan layer that traps the purple crystal violet-iodine complex, so they stay purple. The other colors listed aren’t produced by this pattern, so the pink appearance is the result for Gram-negative bacteria.

Gram staining differentiates bacteria by cell wall structure. After the decolorization step, Gram-negative organisms lose the crystal violet–iodine complex because their thin peptidoglycan layer and outer membrane don’t retain it, so they take up the counterstain. The counterstain is typically safranin, which colors them pink. In contrast, Gram-positive bacteria have a thick peptidoglycan layer that traps the purple crystal violet-iodine complex, so they stay purple. The other colors listed aren’t produced by this pattern, so the pink appearance is the result for Gram-negative bacteria.